Abstract:

Analog data such as text and images are stored in microscopic analog
format on a disk surface capable of maintaining the information for 1000
years or more whereby simple optical magnification will result in one
being able to read the information formed therein. For a disk read by
backlighting, as with microfiche, a photosensitive material is overlayed
on hard metal surface which in turn is formed on a transparent glass or
quartz material. A laser beam is focused on certain desired portions of
the photosensitive material and the exposed material and underlying hard
metal layer etched off to form pits down to the transparent layer
corresponding to the analog information. The resulting disk can then be
used to produce archival copies and distribution copies using hot
embossing or other disclosed techniques.

Claims:

1. A method for forming a negative copy of analog data expressed as three
dimensional surface features formed on substrate, the method comprising
the steps of:forcing a surface of the substrate containing the three
dimensional surface features in contact with a temperature sensitive
polymer under low vacuum and at a temperature higher than the glass
transition temperature of the polymer to thereby transfer a negative of
the three dimensional surface features onto the polymer, said surface and
polymer selected to have to different thermal expansion rates;allowing
the polymer to solidify and shrink during a cooling process to effect a
release of the polymer from the substrate surface; andcoating a high
contrast material over the polymer surface containing the negative of the
three dimensional surface features.

2. The method of claim 1, wherein the high contrast material is selected
from the group consisting of Ti, Ni, Cr or other metal.

3. The method of claim 1, further including the step of forming a scratch
resistant layer over the high contrast material.

4. The method of claim 3, wherein the scratch resistant layer is selected
from the group consisting of diamond-like-carbon (DLC), aluminum oxide,
or silicon dioxide.

5. A method for creating analog data in three dimensions in the surface of
a substrate comprising the steps of:exposing portions of an energy
sensitive glass substrate to directed energy only at predefined pixel
locations corresponding to a reduced image of the analog data to form a
gray level mask pattern;transmitting the gray level mask pattern to a
photoresist layer formed over a substrate; andremoving portions of the
photoresist layer corresponding to the gray level mask pattern to form a
three dimensional representation of the optical data within the
photoresist layer.

6. The method of claim 5, further including the step of depositing a thin
conductive layer over the unremoved portions of the photoresist layer.

7. The method of claim 6, further including the steps of:electrodepositing
an archival quality metal over the conductive layer; andseparating the
conductive layer and electrodeposited layer from the photoresist layer
and substrate.

8. A method for forming a copy of analog data expressed as three
dimensional surface features formed on substrate, the method comprising
the steps of:depositing a thin conductive layer over the three
dimensional surface features of the substrate;electrodepositing an
archival quality metal over the conductive layer; andseparating the
conductive layer and electrodeposited layer from the substrate.

9. The method of claim 8, wherein the step of separating the conductive
layer and electrodeposited layer from the substrate, where the
electrodeposited layer has a different thermal rate of expansion from the
substrate, includes heating the resulting structure.

10. The method of claim 8, wherein the step of separating the conductive
layer and electrodeposited layer from the substrate, where the substrate
is a polymer, includes dissolving the polymer with a ketone.

[0003]This invention relates generally to archival methods and more
particularly to a method and apparatus for physically defining analog
data on a durable medium for long term storage and retrieval using
processing techniques related to those used in semiconductor
manufacturing.

[0004]2. Description of the Prior Art

[0005]Recently, international attention has been focused on the subject of
"Analog Data Storage" i.e. storage of text and images in the analog
format on durable media. The storage format is such that retrieval of the
data only requires magnification of the data onto a viewing screen using
simple optical methods, akin to the process currently used for the
viewing of microfiche.

[0006]Although Microfiche/microfilms are the current accepted method for
analog archival of data, this technology does not serve the purpose of
truly long term archival.

[0007]Microfiche/microfilms require special environmental conditions for
storage of the film. Accordingly, Microfiche is not a durable medium for
archival storage over long periods of time. Secondly,
microfiche/microfilms degrade despite environmental controls in as short
a span as 30-50 years thus requiring re-copying onto new film. Re-copying
leads to some loss of data due to degradation of resolution. Thus the
current technology available for analog data storage for archival
purposes does not provide long-term (>1000 years) storage of data on
durable media capable of withstanding all forms of corrosive
environments. Additionally, storage of data on microfilms becomes
expensive in the long run because of the need for environmental controls
and re-copying at the media end of life.

[0008]Archivists look to analog storage of data because they are concerned
with preservation of text and images over time measured in millennia.
Furthermore, archivists desire that the retrieval of the data should not
be dependent on software or hardware devices such as the case for digital
storage of data.

[0009]The industry has made big strides in digital storage of data where
relatively high densities of data can be stored in various media such as
compact discs, digital versatile discs, storage drives etc. and can be
retrieved at extremely high speeds using computing software/hardware. The
key problem in this form data storage is that it is dependent on the
digital technology available at any given time and suffers from
software/hardware obsolescence in relatively short time spans. A good
example is the 51/4 floppy storage disc and reader both of which were
commonly in use for digital data storage in the 1970s and 80s. Today this
form of storage does not exist and has been replaced by other digital
storage devices capable of higher densities.

[0010]Accordingly, the need remains for an improved method for
implementing archival storage and retrieval of analog data.

SUMMARY OF THE INVENTION

[0011]There are several recognized drawbacks to prior art archival storage
and retrieval methods that the present invention is designed to address.
Objects that the methods and apparatus described herein are designed to
achieve include: [0012]1. Any analog data storage process should be a
very inexpensive process and should use relatively inexpensive apparatus
(writer and reader). [0013]2. The medium on which the data is stored
should be capable of withstanding harsh corrosive environments in order
to ensure its durability over time periods exceeding a thousand years.
[0014]3. The data should be capable of easy retrieval for viewing using
simple optical methods for magnification. This would require the depth of
the features marked on the durable substrates to be optimized in the
range of 400-600 nm deep. This ensures that the data could be read
without having to rely on sophisticated retrieval mechanisms. [0015]4.
The process for storage of the data should be 100% reliable and there
should be no loss of information, quality or resolution during the
archival process. [0016]5. The method of storage of analog data should be
capable of allowing a mixture of text and images to be stored together. A
combination of analog data for long term storage/archival and digital
data for fast rapid access of data could be an ideal archival
implementation. [0017]6. The process and method for storing image
information should be capable of incorporating at least 20 levels of gray
(maximum gray levels distinguishable by human eye) such that grayscale
images can be stored at high resolution. [0018]7. The method should be
extendable to allow storage of color images. [0019]8. The method should
allow creation of archival copies as well as relatively inexpensive
distribution copies. [0020]9. The apparatus used for storing the data on
a durable media should be low maintenance and not require special
facilities to allow easy setup in what could be envisioned as archival
service centers.

[0021]A method and apparatus for analog data storage on durable media
according to a preferred embodiment of the invention is described as
follows. To make this process inexpensive, portable and not requiring any
stringent operating conditions like vibration control, high vacuum, etc.,
a high resolution laser lithography (DWL or "Direct Write Laser") system
is used to create a mask containing the analog (or digital) information
similar to the creation of pattern masks using lithography techniques for
the semiconductor industry for IC fabrication. The use of e-beam
lithography is also contemplated, where e-beam technology (though
generally more expensive) can be used to create pattern masks of very
high data density. As used herein, the process described for data storage
(analog/digital) will be referred to as "permafiche".

[0022]One method for storing analog data on a medium comprising the steps
of first providing a blank permafiche substrate comprising a transparent
substrate, an opaque hard mask layer over the transparent substrate, and
a photoresist layer over the hard mask layer. Portions of the photoresist
layer are then exposed to directed energy only at predefined pixel
locations corresponding to a reduced image of the analog data. Then,
either the exposed portions or the unexposed portions of the photoresist
are removed (depending upon whether the photoresist used is "positive"
where exposed portions are removed, or "negative" where the exposed
portions remain) to thereby uncover corresponding portions of the hard
mask layer located beneath the removed exposed/unexposed portions. The
uncovered corresponding portions of the hard mask layer are then etched
to thereby uncover corresponding portions of the transparent substrate
located beneath the etched portions of the hard mask layer. Finally, the
photoresist layer is removed to yield a copy master having remaining
opaque portions and uncovered transparent portions over which the opaque
hard mask layer has been removed.

[0023]An archival master, using a blank permafiche substrate comprising an
archival substrate such as nickel of any other durable material including
but not limited to diamond, and a photoresist layer, is produced by
similar steps to those above except that the analog information is etched
as a pattern, containing three dimensional surface features, directly
into the archival substrate once the exposed/unexposed portions of the
photoresist are removed.

[0024]Another method for creating analog data in three dimensions in the
surface of a substrate include first exposing portions of an energy
sensitive glass substrate to a dose of directed energy only at predefined
pixel locations corresponding to a reduced image of the analog data to
form a gray level mask pattern. The gray level mask pattern formed within
the glass substrate is then used to transmit the gray level mask pattern
to a photoresist layer formed over a substrate. Finally, portions of the
photoresist layer corresponding to the gray level mask pattern are
removed to form a three dimensional representation of the optical data
within the photoresist layer.

[0025]Yet another method for creating analog data in three dimensions in
the surface of a substrate involves the use of microfilm to make the
analog data etching pattern in the permafiche substrate. The method uses
blank permafiche distribution substrate comprising a photosensitive
polymer layer and a durable substrate layer. The microfilm, including
transparent and/or partially transparent portions, is interposed between
the photosensitive polymer layer of the blank permafiche distribution
substrate and an ultraviolet (UV) source. The photosensitive polymer is
then exposed to the UV source through the microfilm to yield exposed
polymer portions aligned with the transparent or partially transparent
portions in the microfilm. Finally, the exposed portions or the unexposed
portions (depending upon whether the polymer layer is positive or
negative) are removed to thereby uncover corresponding portions of the
durable substrate layer located beneath the removed exposed/unexposed
portions to form three-dimensional features within the distribution
substrate corresponding to the analog data on the microfilm.

[0026]Distribution copies can be formed from the copy master by several
disclosed methods. One such preferred method involves forcing a surface
of the substrate containing the three dimensional surface features in
contact with a temperature sensitive polymer under low vacuum and at a
temperature higher than the glass transition temperature of the polymer.
This results in a negative transfer of the three-dimensional surface
features onto the polymer. The surface and polymer are selected to have
different thermal expansion rates. The polymer is allowed to solidify and
shrink during a cooling process, as when cooling down to room
temperature, to effect a release of the polymer from the substrate
surface. Finally, the shaped polymer surface is coated with a high
contrast material, thereby producing a negative copy of the
three-dimensional surface features.

[0027]Another method for forming distribution copies uses vinyl
polysiloxane that sets and shrinks to effect release from the substrate.
Metal can then be electroplated onto the polymer surface and the metal
separated to form the metal copy of the analog data.

[0028]Finally, analog data can be inscribed on substrates by direct
milling methods such as directed laser beams. Another method, using
e-beams exposes the substrate to a corrosive gas such as xenon difluoride
and then bombarding portions of the exposed substrate with a directed
high-energy electron beam to effect localized etching of the substrate at
the portions.

[0029]The foregoing and other objects, features and advantages of the
invention will become more readily apparent from the following detailed
description of a preferred embodiment of the invention that proceeds with
reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is an illustration of one page of text that contains a
digital code pattern in the footer which, upon digital decoding, displays
the text information.

[0031]FIG. 2 is a schematic diagram of the laser optics and stage system
used to produce an archival substrate in a laser-etching machine,
according to a preferred embodiment of the invention.

[0032]FIG. 3 is a schematic diagram of a laser write head used in the
system of FIG. 2 to etch small details on the blank permafiche substrate.

[0033]FIG. 4 is an illustration of a conversion menu window used by an
operator of the system shown in FIG. 2 to specify the writing parameters
used by the system to form analog (or digital) data on the blank
permafiche substrate.

[0034]FIG. 5 is an illustration of an operator menu used to select various
function of the Operator PC.

[0035]FIG. 6 is a schematic illustration of the electroplating process
used in the electrodeposition intermediate step for forming an "archival
master."

[0036]FIG. 7 is a flow diagram illustrating the process for generating a
"copy master."

[0037]FIG. 8 is a flow diagram illustrating the process for generating an
archival master.

[0038]FIG. 9 is a flow diagram illustrating a preferred process for
generating a "distribution copy" from a copy master.

[0039]FIG. 10 is a flow diagram illustrating an alternate process for
generating a copy from a copy master.

[0040]FIG. 11 is a flow diagram illustrating a second alternate process
for generating a copy from a copy master.

[0041]FIG. 12 is a flow diagram illustrating a preferred process for
generating an archival disk from a copy master.

[0042]FIG. 13 is a flow diagram schematic illustrating a preferred process
for generating an archival master from an archival stamper.

[0043]FIG. 14 is a flow diagram illustrating a process for generating
grayscale analog images using a pit depth variation technique according
to a preferred embodiment of the invention.

[0044]FIG. 15 is a flow diagram illustrating a preferred process for
storing color information from grayscale imagery according to a preferred
embodiment of the invention.

[0045]FIG. 16 is a flow diagram illustrating a method for forming
three-dimensional data structures on a substrate according to an
alternate embodiment of the invention using an energy-sensitive glass
mask substrate.

[0046]FIG. 17 is a flow diagram illustrating a method for forming
three-dimensional data structures on a substrate from an original
archiving microfilm/microfiche transfer process according to the
invention.

DETAILED DESCRIPTION

[0047]Analog data storage using focused ion beam (FIB) technology has been
introduced in the recent years. The focused ion beam system is the
apparatus used for marking special substrates with data in the form of
text or images. Unfortunately, archival methods using focused ion beam
technology would not provide a low-cost, 100% reliable, high volume
archival storage solution due to several key problems. In brief, the
drawbacks to FIB archival methods include very high system cost, a
requirement for use with special facilities such as vibration isolation,
vacuum etc., high maintenance cost, special operator training, long
writing times, expensive support equipment for post-processing, and
transfer of information into archival quality substrates. Finally and
perhaps most importantly, focused ion beam systems do not ensure 100%
accurate transfer of data to the storage medium and are thus not suitable
for high volume, production environments.

[0048]The FIB is a specialized tool, which has some key applications in
the semiconductor industry such as circuit edit, mask repair and failure
analysis. The FIBs require constant monitoring to ensure smooth operation
and require setting up of service agreements with the manufacturer to
maintain the tool. When the FIB is used as a writer to transfer data in
the form of text to the substrate, the depth of features of the
transferred pattern are in the sub-60 nanometer range. There are two key
disadvantages to having such shallow features: (1) specialized optics
need to be incorporated into the reader in order to resolve the data, and
(2) special care is needed to avoid accidentally losing the data from
minor scratching.

[0049]Another disadvantage of focused ion beam lithography techniques for
archival purposes is that writing on materials including a transparent
base layer (for backlit viewing as with standard microfiche machines) is
extremely complicated.

[0050]The invention contemplates advances made in direct write laser
(DWL), focused ion beam (FIB) and e-beam technologies for use as the
directed energy used to form the three dimensional structures
illustrating the analog data. Currently, however, using a DWL system
instead of using other sophisticated equipment such as a focused ion beam
systems incurs several advantages. In recent years analog data storage
using focused ion beam technology has gained a lot of attention although
a complete low cost solution using this technology is yet to be
developed. Several key problems that stand in the way of making this a
viable technology are described elsewhere in this document.

[0051]Analog data stored on suitable media using the DWL technique is read
by magnification of the patterns on the analog media using simple optical
methods and displaying the pattern on a screen (computer screen, TV
monitor, or direct viewing of the image through a binocular lens).
Although the analog form of data storage is the key requirement of
archival applications, an additional feature that can be incorporated
into the data that is stored on the durable media is the addition of
digitally encoded information such that the data can be rapidly accessed
using appropriate decoding software for day to day access and
distribution of information that is stored on the "permafiche".

[0052]The illustration shown in FIG. 1 shows how a page archived using
methods and systems taught by the present invention might appear with
both analog and digital information. Given a page 20 of text (or image)
that needs to be stored on the permafiche media, the analog text (or
image) portion 22 of the page 20 is converted into a digitally encoded
matrix 24 that is present as a pattern in the footer section of the page.
When the page is stored on the permafiche media, the digital code is also
transferred to the permafiche substrate as a pattern. Using appropriate
software decoding scheme, the page can be rapidly accessed after the
image is displayed on the reader. Examples of digital patterns types that
can be reproduced from an analog source are varied and well known and are
thus are not described in detail here.

[0053]The Direct Write Laser (DWL) used to implement the invention is a
high precision instrument using raster technology to image on various
substrates like silicon, glass, film, or other photosensitive type
plates. In addition to its use in the present invention, the DWL can be
used to produce masks for semiconductor processing and direct writing,
integrated optics, lead frames, flat panel displays, shadow masks, and
any application where high precision, high-resolution images must be
produced. The DWL can accommodate various laser types and write heads and
allows the user access to parameter settings to optimize the use of these
devices on various media.

[0054]The Direct Write Laser system used in the preferred embodiment of
the invention is available in various models from companies such as
Applied Materials, Heidelberg Instruments Mikrotechnik, and Micronics to
name a few. The system used in the preferred embodiment includes the
following features: [0055]Metal cover for light and dust protection;
[0056]Heavy granite support system with air cushions for vibration
isolation; [0057]Air-cooled HeCd Laser; [0058]Optical beam-system;
[0059]Precision stage system consisting of the following components:
[0060]6'' XY stage with linear motors; [0061]Special wafer/mask chuck
designed for single plates; and [0062]Two axis interferometric
positioning system. [0063]Airgauge autofocus system; and [0064]Complete
electronic control system.It is understood, of course, that the invention
is not limited to the manufacturer or the type of laser system.

[0065]The DWL produces excellent quality, high resolution images using a
high integrity mechanical structure, a precise position measurement
system, high speed positioning control system, and beam position control.
A simple schematic diagram of the system is shown in FIG. 2, however such
a system is well known in the art by those who produce such equipment for
use in semiconductor processing and thus is not described in detail here.
In the DWL the optics remain fixed and the media is moved. With this
design the optic path remains constant thereby eliminating any need to
refocus or recalibrate as would be required in a system where the optical
write head is moved. This simple design makes the system reliable and
creates images with consistent high quality. This technology ensures
position accuracy, orthogonality and quality of image. A combination of
deflection and stage motion as well as multiple write heads could be also
used to speed up the process of writing.

[0066]The DWL system can be used for laser pattern generating on various
substrates using photoresist. Processes are described below with
reference to FIG. 7 and later. The effective write grid of the DWL used
is 100 nm with a minimal structure size of 2 μm using a 10 mm write
head. In an active write area of 140 mm×140 mm, the write time for
a two-inch square area is approximately six hours. The stage system upon
which the substrate is mounted has a positional resolution of 40 nm.

[0067]The DWL offers an interchangeable write head with a selection of
multiple lenses. An example of one type of write head used is shown in
FIG. 3, although such heads are well known in the semiconductor
mask-making industry and thus are not described in detail here. Each lens
offers a different resolution and feature size to meet the needs of the
application. The 10 mm write head used in the preferred embodiment has an
8 μm depth of focus, a 5 mm coarse z-range, a 70 μm fine z-range, a
stability of less than 200 nm and is mounted approximately 100 μm from
the substrate (shown by "D" in FIG. 3). Only one lens can be used at any
one time per mount, although it is understood that multiple write head
mounts can be used in parallel to speed up the write process as described
further below. It typically takes less than ten minutes to change from
one lens to another.

[0068]The laser system used is a special low noise HeCd-laser suitable for
resist exposures. The laser used in the DWL generates coherent light
having a wavelength of 442 nm, and operates with a power of 20 mW for a
typical lifetime of greater than 4000 hours. The optics used in the DWL
includes a lens system with highly refractive mirrors, and an
acousto-optic modulator system.

[0069]The electronic components are located in an external nineteen-inch
rack wired according to VDE 110 including the following components:
VME-bus processor system, a 68040 processor available from Motorola, a
standard 4 GB SCSI hard disk drive, OS-9 operating system, a pixel
generator, a thin-Ethernet port for local area network (LAN) running
TCP-IP, modulator electronics including real-time software for data
decompression and exposure control, and control electronics for the state
and auto focus system. Again, the specific components are not of
particular importance and one skilled in the art would recognize that
many other configurations are possible to operate the DWL.

[0070]The control PC used in the system utilizes a 500 MHz or faster
processor, 128 MB of RAM, a display device, and is operates using Windows
NT/Linux or other suitable system. Data conversion software operates on a
Conversion PC to convert formation data, such as the conversion menus
shown in FIG. 4, into machine driver code for operating the DWL. The
workflow for the system would be as follows: After generating the design
with a appropriate Design software, the design will be converted on the
Conversion PC into the HIMT internal format called LIC. The LIC format is
a highly compressed machine format, which will be used by the DWL system
for the exposure. After conversion on the Conversion PC the LIC will be
send to the DWL system controller via Ethernet. The conversion software
provides an easy to use and powerful graphical interface with additional
functions like image reversal (positive/negative), biasing, scaling etc.

[0071]The Operator PC runs under Windows NT and includes a system control
menu shown in FIG. 5.

[0072]There are several options that can be used with the DWL system
including a metrology and alignment system, a front to back side
alignment system, additional write heads, a larger stage for increased
active write area, and an environment chamber including flowbox.

[0073]The metrology and alignment system is used for multi-layer exposures
and metrology measurements and includes the following components: Image
Processing (IPC), two camera system, white light illumination and image
software (FIPS). The alignment accuracy with a 4 mm lens is on the order
of 250 nm. Using front- to back-side alignment with a CCD camera system
for three to six inch wafers, the alignment accuracy is on the order of 2
μm.

[0074]The system has an exchangeable write head system. The operator can
easily exchange each write head within 10 minutes. Exchanging the write
head changes the system's performance specifications such as throughput,
resolution, etc. The following are examples of different write heads that
can be used:

[0075]Another option is to increase the size of the stage upon which the
substrate is mounted from six inches to eight inches, thereby increasing
the active write area to 170 mm by 185 mm.

[0076]Finally, the system can be fitted with a flowbox, which provides the
environment of the system with a laminar airflow, a constant temperature
and clean air. The airflow is adjustable. The temperature inside the
environment chamber is typically 2° higher than the rated value of
the environment outside of the chamber. The airflow is adjustable between
flow rates of 0.3-0.5 m/s, with a temperature stability of ±1°
C., at a class 10 air quality.

[0077]We will now proceed with a description of the preferred processes
for manufacturing two types of archival substrates, termed a "copy
master" substrate (sample process of manufacture shown in FIG. 7) and an
"archival master" substrate (sample process of manufacture shown in FIG.
8).

Formation of the Copy Master

[0078]The copy master substrate incorporates a base substrate 30, hard
metal (hard mask) 32, and photoresist layer 34 forming a blank permafiche
substrate 36. The base substrate 30 is a transparent material, preferably
formed of glass, quartz or sapphire having a thermal conductivity very
similar to the metal mask placed on top of the base substrate in a later
process step. It is also contemplated that the base substrate is a
plastic or polymer exhibiting transparent optical properties. The base
transparent substrate has an approximate thickness of between about 0.1-2
mm, however this thickness is not a very critical parameter for the
archival methods described.

[0079]In forming the blank permafiche substrate 36 shown in step A, a hard
mask 32 is deposited on the base substrate as by sputtering, evaporation,
etc. to form a low stress layer "Hard Mask". The hard mask 32 is
preferably chrome or iron oxide but can also be made from other metals,
metal oxides, oxides, nitrides, and carbides as listed below:
[0080]Metals: Chrome, Tantalum, Gold, Platinum, Titanium, Nickel, Iron,
Tungsten etc [0081]Metal Oxides: Iron oxide, chromium oxide, Tungsten
oxide, Titanium oxide etc. The main advantage of using metal oxide or
oxides over metals is one can obtain much finer features and smoother
wall profiles, mainly because oxides are amorphous materials and the
feature/profiles are not limited by the grain size of the material.
[0082]Oxides: Silicon dioxide etc [0083]Nitrides: Silicon Nitride,
Tungsten nitride, Tantalum nitride etc. The nitrides also has similar
advantages as oxides, and because of its high density (very close to its
pure metal form) one can use nitride to build X-ray masks. [0084]Others:
Diamond like carbon (DLC), Diamond films etc.The typical thickness of the
hard mask varies from 50 nanometers to a few microns.

[0085]A photoresist layer 34 is then deposited on the hard mask layer, as
by spin or spray coating, to form a thin uniform layer of photosensitive
polymer on top of the hard mask. The polymer is the top most layer on
which the data is first written using the DWL system according to the
invention and as shown in FIG. 7. A mono-layer adhesion layer could be
used prior to spin coating or spray coating the photoresist layer over
the hard mask layer. The adhesion layer helps in providing a good bonding
surface for the photoresist layer to the hard mask layer. Typical
adhesion material is HMDS, although it is understood that others
knowledgeable in the art would recognize other materials that would work.
Typical resist thickness is between 400-550 nanometers, but one can use
thinner or thicker photoresist layers anywhere from a few nanometers to a
few microns.

[0086]Typical photoresists used are both positive types and negative
types. The preferred positive types are type AZ1518 available from AZ
Electronic Materials of Somerville, New Jersey, and the Shipley 1800
series available from Shipley Company of Marlborough, Massachusetts. It
is understood that other AZ series resists, and other positive resists
from Shipley can be used without departing from the teachings of the
invention. Available negative photoresist includes SU8, developed by IBM
and supplied by Silicon Resources of Chandler, Arizona with other
negative resists also available from AZ Electronic Materials and Shipley.
In our current application, the preferred photoresist is positive,
photoresist AZ1518 and Shipley 1800 series, and the thickness of the
resist layer is 520 nanometers.

[0087]Referring still to FIG. 1, shown are the method steps for generating
a copy master. In step 1, copy master substrate 36 (incorporating the
transparent base substrate, hard mask layer, and photoresist layer
described above) is mounted to the stage of the DWL system. The
photosensitive layer (top layer) is then exposed in step B by the laser
38 only at predefined pixel locations 40, 42 that describe the pattern of
the data being written. This pattern is determined according to the LIC
format (driving the DWL Control PC) that is generated by converting the
parameters within the conversion menu (FIG. 4) using the data conversion
software operating on the Conversion PC.

[0088]The process for using ion beam or e-beam energy sources is identical
to the one that was provided for the laser writing process. The only
difference is in the beam that is used to expose the photoresist. Here we
are providing the typical parameters that we used to expose the
photoresist using electron and ion beam:

Electron Beam Writer:

[0089]Accelerating voltage: 2-30 kV (ideal: 30 kV)

[0090]Beam current: 5 picoamps to 10 nanoamps (ideal: 80 picoamps)

[0091]System Manufacturers: JEOL, Applied materials, Leica, Hitachi

Ion Beam Writer:

[0092]Accelerating voltage: 5-120 kV (ideal: 30 kV)

[0093]Beam current: 4 picoamps to 20 nanoamps (ideal: 10 picoamps)

[0094]System Manufacturer: FEI, LEO, Hitachi, Seiko and Schlumberger

[0095]After the laser exposure step, the copy master substrate 36 is
exposed to a developer in step C to remove either the exposed or
unexposed regions 42, 44 depending upon whether the developer is positive
or negative. The developer could be either sprayed on the substrate or
the substrate could be dipped in a container filled with the developer
solution. [0096]Case 1: If the photoresist layer 34 is positive in
nature as shown in FIG. 7, the laser exposed pixels or regions 42, 44
will be removed by the developer to reveal exposed portions 46, 48.
[0097]Case 2: If the photoresist layer 34 is negative in nature (not
shown in FIG. 7), the laser exposed pixels or regions 42, 44 will remain
intact and the rest of the unexposed area will be removed by the
developer.

[0098]The developed copy master substrate 36 is next subjected to a low
temperature bake step to harden the photoresist layer. Typical heating
parameters are 150° C. for 1-2 minutes. Using the photoresist as
the soft mask the next step is to transfer the pattern or information
from the soft mask to the hard mask, thereby forming three-dimensional
features such as cavities 50, 52. Two transfer techniques could be used
with the final results shown in step D: (1) use of wet chemistry, and (2)
dry etching for very precise control of the etch rate and feature wall
profile. These techniques are described in further detail below:

Etching Technique #1:

[0099]Currently we are using Chrome as our hard mask material and here is
the procedure we are following to transfer the pattern generated on the
photoresist to the chrome layer (hard mask layer). The procedure
described below uses a wet chemical etching process:

First dissolve the acid in 700 ml of DI water. Then add the Ceric ammonium
nitrate and stir for about ten minutes until the solution is almost
clear. Now add the remaining water to make 1000 ml. If the water has not
been acidic before the adding the ceric ammonium nitrate then a
precipitate is formed which cannot be removed by filtering. The etchant
should be filtered before use. Any drying of blanks during processing
should be avoided. Rinse at least 2 minutes in running DI water or fifty
seconds DI water spray after etching.

[0101]Etching Technique #2: The above process involves wet chemistry and
due to isotropic etching nature of wet chemistry very sharp vertical wall
profile may not be obtained. But for our application, wet chemistry is an
ideal solution. It is cheap and fast and does not require any additional
expensive equipment. But one can also use dry etching process to obtain
near vertical wall profile and much tighter control on the process and
cleanliness. The typical dry etch chemistry as follows for etching of
chrome: (The dry etching process is a well known technique and is not
being explained here)

[0102]After the pattern or information is transferred from the soft mask
to the hard mask, the soft mask (photoresist layer) is stripped off in
step E either using wet chemistry or dry chemistry. Typical wet chemical
stripping agents are ketone based while for dry chemistry they are
oxidizing gases. Thereafter the copy master 54 is ready. One can then
generate multiple distribution copies from the copy master (FIGS. 9-11)
or generate the final archival master for long-term storage from the copy
master (FIG. 12).

Formation of the Distribution Copies

[0103]Distribution Copies are relatively inexpensive permafiche materials
formed using the copy master 54 as a template, as shown in FIGS. 9-11.
Three schemes for forming the distribution copies are detailed below:

[0105]Under a first scheme, shown in FIG. 9, the "copy master" 54 is
aligned and placed in contact with the distribution substrate 60 as shown
in FIG. 9, Case 1. Alternately, the copy master can be aligned and placed
in contact with the distribution substrate 60 as shown in FIG. 9, Case 2.
The substrates 62 for creation of distribution copies consist of any
robust/durable material such as metal or polymer with a thin coating of
photosensitive polymer. The polymer 64 type chosen is appropriate for
exposure under the wavelength range of the UV source, having an
approximate thickness of 400 nm. The choice of the metal or polymer for
the substrate is based on the thermal expansion coefficient, which should
be similar to the thin coating of photosensitive polymer coating
(photoresist) and also should provide good adhesion property to the
photoresist. The preferred photoresist polymer used is polymethyl
methacrylate, also known as PMMA, although it is understood that other
polymers can be used.

[0106]In steps A1 or A2, a broad UV source or a UV scanner 66 is used to
expose the photosensitive polymer 64 on the distribution substrate 62.
The copy master 54, acting as the contact mask, serves the purpose of
allowing exposure of the photosensitive polymer only in the regions 68,
70 directly beneath the open regions 50, 52 of the chrome pattern.

[0107]After the exposure step, the distribution substrate 62 is exposed to
the appropriate developer and in step B the UV exposed regions 68, 70 of
the photosensitive polymer 64 are dissolved out (as in the case when a
positive photoresist is used as the photosensitive polymer). Using a
negative photoresist will of course result in the unexposed regions being
dissolved out. The resulting structure includes exposed regions 72, 74
through the photosensitive polymer 64.

[0108]Finally, a thin layer of metal such as Ti, Ni, Cr or other metal is
sputtered over the patterned distribution substrate 62 in order to form a
contrast enhancing layer 76 and provide a high contrast of features for
viewing. Following that a scratch resistant material such as
diamond-like-carbon (DLC), aluminum oxide or silicon dioxide is sputtered
over the metal layer to form scratch resistant layer 78 of the final
distribution copy 79.

[0110]Under a second scheme, shown in FIG. 10, the copy master 54 is
aligned and placed over a polymer material 80. These are then enclosed in
step A in a low vacuum chamber that is maintained at a temperature range
higher than the glass transition temperature range of the polymer
(180-200° C.), whereby the distribution substrate 80 consists of a
durable temperature sensitive polymer of appropriate thickness to ensure
ease of handling. An embossing technique is used where a uniform load is
applied on the copy master 54 in contact with the polymer 80 in step B.
This causes the transfer of the pattern of the copy master 54 onto the
polymer 80 whereby the polymer now is embossed with a pattern that is an
exact negative of the copy master. In the example shown, for instance,
voids such as open regions 50, 52 in the patterned hard mask layer 32 of
the copy master 54 form complementary peaks 82, 84 in the polymer.

[0111]To disengage the copy master 54 from the now-embossed distribution
copy polymer substrate 80, a heating process is used to take advantage of
the thermal expansion (and contraction) properties of the materials used.
After completion of the high temperature hot embossing technique, the
copy master 54 with attached patterned polymer 80 are withdrawn from the
low vacuum chamber and cooled to room temperature in step C. During the
cooling process an approximate 5% shrinkage of the polymer 80 results
which is instrumental in releasing the copy master 54 from the polymer.

[0112]Once removed, a thin layer 86 of metal such as Ti, Ni, Cr or other
metal is sputtered over the patterned distribution substrate in step D in
order to provide high contrast of features for viewing. Following that a
scratch resistant material 88 such as diamond-like-carbon (DLC), aluminum
oxide or silicon dioxide is sputtered over the metal layer to form the
final distribution copy 89.

[0113]The big advantage of this method lies in its flexibility, the low
internal stresses and high structural replication accuracy due to the
small thermal cycle (approximately 40° C.), so that structural
replications in the nanometer-range are possible.

[0114]Scheme 3 (FIG. 11)

[0115]The methods in FIG. 11 are similar to those in FIG. 9, the
difference being the type of distribution substrate 90. The distribution
substrate is transparent and consists of a glass or quartz substrate 92
with a thin layer of metal or metal oxide 94 over which is formed another
thin layer of photosensitive polymer 96. The copy master is aligned and
placed over the distribution substrate as illustrated in FIG. 11 (Case 1
or Case 2). Exposure steps A1 or A2 and developing step B are similar to
that described in FIG. 9. The is also an etching of the hard mask 94 to
form voids 100, 102 in step C and photosensitive polymer layer 96 removal
in step D similar to that used in forming the copy master. This is then
followed in step E by application of a scratch resistant coating 98 over
the patterned metal or metal oxide layer over the glass or quartz
substrate. Unlike the duplication process described in FIG. 9, the FIG.
11 process describes a duplication process for creation of a transparent
distribution copy. The distribution copy 99 formed using FIG. 11 would
thus result in a disk where low-cost back illumination optics, such as
used in the conventional microfiche industry, can be used for reading the
information on the disk.

[0116]Scheme 4

[0117]A low viscosity vinyl polysiloxane impression material is used to
produce distribution copies from archival master or from copy master.
Features as small as 1-2 microns have been replicated using the
impression material. The key features of the vinyl polysiloxane material
are as follows:

[0118]Tasteless and odorless. (Dentists use similar polymer material for
dental impression)

[0119]Electroplate able

[0120]Does not absorb moisture

[0121]Can replicate accurately features as small as 1-2 microns

[0122]It does not discolor

[0123]No shrinkage after it is cured

[0124]Long shelf life ˜2 years

[0125]Take about 2-4 minutes to generate a replicate from the master disk
manually

[0126]Does not require any heating

[0127]Does not require release agent

[0128]The vinyl polysiloxane is a two-part mixture consisting of a base
and an accelerator. A predetermined amount is mixed either manually or
using an automated syringe type dispensing system. The master or copy
master disk from which the distribution copies are to be generated is
cleaned with alcohol and blown-dry it with nitrogen gas. The low
viscosity impression material is then poured on the master disk or copy
master. Using a flat glass plate, one then applies a uniform pressure to
primarily flatten the back surface of the distribution copy. The
impression material is then allowed to set for approximately 2 minutes
during which time the impression material shrinks by ˜0.05% which
helps in easy disengagement of the impression material from the master or
copy master disk. In this process there is no need for release agent,
there is no residue, nor there is any reaction with the metal in the
master or copy master disk. Since the polymer is electroplatable,
distribution metal copies may be generated from the polymer copy.

Formation of the Archival Master

[0129]The "archival master" is a disk formed of a base archival quality
substrate on which pits are formed for storing information in analog
form. The archival master can be formed directly from the DWL process
shown in FIG. 8, or by using a copy master or distribution copies as
disclosed in FIG. 12.

[0130]The preferred archival metal substrate 104 is generally pure nickel,
but various other durable materials could be used including but not
limited to diamond, sapphire, or pure nickel embedded with fine diamond
powder. The introductions of angstrom size diamond particles in the
plating bath during electroplating (FIG. 6) will help reduce stresses
during the plating process and also increase wear and thermal resistance
properties. Currently nickel has been identified as the material of
choice for the archival substrate but it is understood that more
experiments will be conducted in the future to compare nickel with a few
other metal alloys and even diamond substrates.

[0131]A photoresist layer 106 is then deposited on the archival metal
substrate layer, as by spin or spray coating, to form a thin uniform
layer of photosensitive polymer on top of the substrate. The polymer is
the top most layer on which the data is first written using the DWL
system according to the invention and as shown in FIG. 8. A mono-layer
adhesion layer could be used prior to spin coating or spray coating the
photoresist layer over the archival metal substrate. The adhesion layer
helps in providing a good bonding surface for the photoresist layer to
the substrate. Typical adhesion material is HMDS, although it is
understood that others knowledgeable in the art would recognize other
materials that would work. Typical resist thickness is between 400-550
nanometers, but one can use thinner or thicker photoresist layers
anywhere from few nanometers to few microns. The preferred photoresist
used is similar to the types detailed in the previous section above for
formation of the copy master.

[0132]Referring still to FIG. 8, shown are the method steps for generating
an archival master. In step A, the archival master substrate 108
(incorporating the archival metal substrate 104 and photoresist layer 106
described above) is mounted to the stage of the DWL system. The
photosensitive layer (top layer) 106 is then exposed by the laser 38 only
at predefined pixel locations 40, 42 that describe the pattern of the
data being written. This pattern is determined according to the LIC
format (driving the DWL Control PC) that is generated by converting the
parameters within the conversion menu (FIG. 4) using the data conversion
software operating on the Conversion PC. After the laser exposure step B,
the archival master substrate 108 is exposed to a developer in step C.
The developer could be either sprayed on the substrate or the substrate
could be dipped in a container filled with the developer solution.

[0133]Case 1: If the photoresist layer 106 is positive in nature as shown
in FIG. 8, the laser exposed pixels or regions 42, 44 will be removed by
the developer to reveal exposed portions 46, 48.

[0134]Case 2: If the photoresist layer 106 is negative in nature (not
shown in FIG. 8), the laser exposed pixels or regions 42, 44 will remain
intact and the rest of the unexposed area will be removed by the
developer.

[0135]The developed copy master substrate 108 is next subjected to a low
temperature bake step to harden the photoresist layer. Typical heating
parameters are 150° C. for 1-2 minutes. Using the photoresist 106
as the soft mask, the next step is to transfer the pattern or information
from the soft mask to the archival metal substrate in step D. The exposed
portions 46, 48 then result in etched portions 50, 52. Three techniques
could be used: (1) use of wet chemistry, (2) dry etching for very precise
control of the etch rate and feature wall profile, and (3) reverse
electroplating. These techniques are described in further detail below:

Etching Technique #1: With nickel as the preferred archival quality
material, the first technique followed to transfer the pattern generated
on the photoresist to the nickel layer (archival quality metal substrate)
uses a wet chemical etching process:

[0136]The standard formulation for the nickel etch uses a mixture of
H2SO4 (Sulfuric acid) and H2O2 (Hydrogen Peroxide) at
a temperature of 140° Fahrenheit.

[0137]Etching Technique #2: The above process involves wet chemistry and
due to isotropic etching nature of wet chemistry very sharp vertical wall
profile may not be obtained. But for our application, wet chemistry is an
ideal solution. It is cheap and fast and does not require any additional
expensive equipment. But one can also use dry etching process to obtain
near vertical wall profile and much tighter control on the process and
cleanliness. The typical dry etch chemistry is as follows for etching of
nickel:

[0138]Etching Technique #3: Reverse electroplating involves a process
where the electroplating set up has a reverse bias applied to the sample
(the archival master 108) i.e. the sample is made into the positively
charged anode. [The "electroplating" or "electrodeposition" basics have
been described further below.] The negatively charged cathode that will
complete the D.C. circuit could be composed of any conductive metal. The
archival master is submerged in the electroplating bath with the
patterned soft mask facing downwards. The cathodes may be placed at the
sides or bottom of the tank. During reverse electroplating, regions of
the patterned archival master with the Nickel exposed will get etched out
by the movement of Nickel into the plating berth. The depth to which the
exposed Nickel gets etched is controlled by the bias applied, the plating
bath chemistry, and the time. The plating bath chemistry has been
detailed further below in the section describing electrodeposition.

[0139]After the pattern or information is transferred from the soft mask
to the archival metal substrate, the soft mask (photoresist layer 106) is
stripped off in step E either using wet chemistry or dry chemistry.
Typical wet chemical stripping agents are ketone based while for dry
chemistry they are oxidizing gases. Thereafter the archival master 110 is
ready.

[0140]FIG. 12 illustrates the method for forming an archival master using
an electroplated negative archival stamper as an intermediate step. To
generate the archival master we can start with either the copy master 54
or distribution copies 79, 89, or 99 generated by any one of the three
schemes described previously in the section above. A conductive metal
layer is first deposited over the patterned copy master or distribution
copy using sputtering, evaporation or electro less technique. This forms
the "seed" layer (conductive layer).

[0141]Using an electro-deposition technique, an appropriate thickness of
metal 112 is deposited over the seed layer. The type of metal chosen for
the fabrication of the "mother" (also known as the "archival stamper")
should be different than the metal chosen for generation of the archival
master in order to take advantage of a difference in thermal expansion
coefficients of two different metals. In a subsequent step, one then
separates the "mother" from the copy master 54 or distribution copy 99
generated by FIG. 11 using heating, thereby taking advantage of the
difference in thermal expansion coefficients. One can separate the
"mother" from the distribution copy generated by methods shown in FIGS. 9
and 10 by using a ketone based solvent. Note that the "mother" created by
electrodeposition 114 over the distribution copy 89 generated by FIG. 10
will have reverse pattern to the "mother" 112 generated by the copy
master and distribution copy 79, 99 generated by FIGS. 9 and 11.

[0142]The "mother", also called the archival stamper, is now used in FIG.
13, step A for generation of the archival master. In step B of FIG. 13,
the metal forming the archival metal substrate 110 is electrodeposited on
the "mother" (either metal pattern 112 or negative metal pattern 114).
The archival master 110 is then separated in step C from the archival
stamper 112 or 114 by heating, thereby taking advantage of the
differences in thermal expansion coefficients of the two metals. The
peaks of the archival stamper, such as peaks 101, 103 in metal pattern
112, result in valleys or pits in the electrodeposited archival master
110, such as corresponding pits 105, 107. Likewise, the pits of the
archival stamper, such as pits 109, 111 in metal pattern 114, result in
peaks formed in the electrodeposited archival master 110, such as peaks
113, 115. The raw archival master 110 in step D (a negative of the
"mother") can then be coated with a scratch resistant coating 116 if
needed in step E.

Electrodeposition

[0143]Electrodeposition can be defined as the deposit of a very thin layer
of metal "electrolytically" to a base metal or other substrate materials
on which a metal "seed layer" has been deposited by sputtering,
evaporation or electro-less technique. Electrodeposition is done in a
liquid solution called an "electrolyte" (fig. A in FIG. 6), also known as
a "plating bath". The plating bath is a specially designed chemical bath
that has the desired metal (Brass, Cadmium, Copper, Gold, Silver, Tin,
Zinc, Chromium, Nickel-Cobalt or other metals) dissolved as microscopic
particles (positive charged ions) suspended in solution. The plating bath
solution serves as a conductive medium and utilizes a low D.C. voltage
(direct current). The object that is to be plated is submerged into the
plating bath and a low voltage D.C. current is applied to the bath.
Generally located at the center of the plating bath, the object that is
to be plated acts as a negatively charged "cathode" (fig. B in FIG. 6).
The positively charged "anodes" (fig. C in FIG. 6) that will complete the
D.C. circuit are carefully positioned at the edges of the plating tank. A
power source known as a "rectifier" (fig. D in FIG. 6) is used to convert
A.C. power to a carefully regulated low voltage D.C. current.

[0144]The resulting circuit channels the electrons into a path from the
rectifier to the cathode (object being plated), through the plating bath
to the anode (positively charged) and back to the rectifier. Since
electrical current flows from positive to negative, the positively
charged ions at the anodes flow through the plating bath's metal
electrolyte toward the negatively charged cathode. This movement causes
the metal ions in the bath to migrate toward extra electrons located at
the cathode's surface outer layer. By means of electrolysis, the metal
ions are taken out of solution and are deposited as a thin layer onto the
surface of the object.

[0145]This process is called electrodeposition. Theoretically, the
thickness of the electrodeposited layer deposited on the object is
determined by the time of plating, and the amount of available metal ions
in the bath relative to current density. The longer the object remains in
the D.C. activated plating bath, the thicker the electrodeposited layer
will become. The inherent shape and contour of the object can affect the
thickness of the plated layer. Metal objects with sharp corners and edges
will tend to have thicker plated deposits on the outside corners and
thinner deposits in the recessed areas. This occurs because the D.C.
current flows more densely around the outer edges of an object than the
less accessible recessed areas. With rare exception, electroplating
processes will not conceal preexisting surface blemishes such as
scratches, dents, or pit. In fact, the plating process has a tendency to
make most surface imperfections even more noticeable. It is therefore
necessary to remove any undesirable surface marks prior to the
electrodeposition process.

Storing Grayscale Data in Analog Form

[0146]Two methods are described herein for storing grayscale data in
analog format on disks such as those described above: (1) Pit Depth
Variation, and (2) Dithering.

[0148]Grayscale information can be patterned onto a photosensitive polymer
by exposing the polymer to a variation in the intensity or the time of
exposure to the laser beam. Start with any suitable substrate 120, which
has a coating of photosensitive polymer 122. The photosensitive polymer
122 is then exposed at desired pixel locations 124, 126 and 128 to a
controlled amount of directed energy as from a direct write laser (DWL).
In FIG. 14, pixel 124 is exposed to a low amount of total energy as
determined by the exposure intensity I1, the exposure time T1,
or some combination therebetween. Pixel 126 is exposed to a medium amount
of energy as determined by the exposure intensity, the exposure time
T2, or some combination therebetween. And pixel 128 is exposed to a
relatively high amount of total energy as determined by the exposure
intensity I3, the exposure time T3, or some combination
therebetween where I1<I2<I3 and
T1<T2<T3. The grayscale pattern, consisting of
varying pit depths, is transferred in step A to the photoresist by either
dynamically varying the intensity of the laser beam or by varying the
exposure time of the laser beam.

[0149]The substrate is then put through a develop step B (for positive
photoresist the laser exposed regions are dissolved away) which exposes
the grayscale pattern in the form of varying depths of pits. Pixel 124,
receiving the least amount of directed energy, results in formation of a
pit 130 formed to depth D1 in photoresist 122. Pixel 126, receiving
a medium amount of directed energy, results in formation of a pit 132
formed to depth D2 in photoresist 122. Pixel 128, receiving the most
amount of directed energy, results in formation of a pit 134 formed to
depth D3 in photoresist 122 where D1<D2<D3.
When viewed with an optical microscope, deeper pits (e.g. pit 134) appear
darker than shallower pits (e.g. pit 130) to thus yield a gray scale-type
perspective for the viewer. A copy master or archival stamper (note:
these will be in the reverse format) can be created from this substrate
by using electrodeposition technique or hot embossing technique described
previously. Distribution copies or the archival master can be generated
from the copy master or archival stamper using the process described
above. Using this process to generate patterned substrates containing
grayscale as well as text information requires frontside illumination for
reading the data. The reader should be capable of resolving at least 20
levels of gray to display the image information at appropriate
resolution.

[0150]Grayscale Data Using Dithering Scheme

[0151]The word dither refers to a random or semi-random perturbation of
the pixel values. It is possible to display a grey-level image in a
bilevel device such as monochrome displays and many hardcopy printers by
using a technique called image dithering. It consists of mapping the
original greyscale image into a binary image. As our eyes perform a
spatial integration, it is possible to achieve reasonable results by
using a mapping strategy where the gray-intensity values are converted to
density of black pixels.

[0152]One of the results of dithering is a slight loss of image
resolution. This happens because the grayscale blocks are formed from
clusters of pixels. For example, an eight-level grayscale requires a
3×3 grid of pixels while a 16-level grayscale requires a 4×4
grid of pixels to represent all levels of brightness. The above patterns
are called clustered-dot dithering, because each subsequent pattern turns
on or off a pixel that is touching a previously modified pixel. Another
form of ordered dot dithering is dispersed-dot dithering where, if
possible, the next pixel to be modified is not touching a previously
modified pixel. An image modified by either of the dither techniques
contains enough information to be usable, but banding and moire effects
can occur.

[0153]To overcome these issues, a third dithering method known as the
"error diffusion" dither. Error diffusion dithering does not generate
banding or moire effects. It also takes into account the grayscale values
of the preceding pixels when determining the proper dither value.
However, error diffusion dithering modifies pixels in both the current
line and the subsequent line. As with standard threshold dithering, error
diffusion dithering can be used to create color dithered images as well
as bi-level images.

[0154]Use the appropriate dithering scheme to produce the grid of pixels
that correspond to the grayscale or color image broken down into its RGB
components. The process of transferring a color image to the
photosensitive polymer using the DWL system is described in the next
section. The pattern is then transferred to the photosensitive polymer
over a suitable substrate using the process described in the previous
section.

Reproducing Color Information

[0155]In a first step, illustrated in FIG. 15, one can use an appropriate
algorithm to convert the color image into its RGB components in step A.
This generates three separate images: one each for the red 140, green 142
and blue 144 components. The pattern for the three images is then
transferred to the DWL system. Each of the red, green and blue images can
now be treated as a grayscale image that will be patterned onto the
photosensitive polymer (photoresist) over suitable substrate in step B.
Transfer the pattern to the photoresist using either the pit depth
variation technique or the dithering technique. Note that the pattern
containing the color image occupies three times the space that a
grayscale image would occupy on the substrate since three images (for
each of the RGB components) have to be written on the substrate

[0156]The substrate is then put through the develop step (for positive
photoresist the laser exposed regions are dissolved away). A copy master
or archival stamper can be created from this substrate by using
electrodeposition technique or hot embossing technique, described
previously. Distribution copies or the archival master can be generated
from the copy master or archival stamper using processes described
earlier.

[0157]Using this process to generate patterned substrates containing color
as well as text information requires frontside illumination for reading
the data if pit depth variation technique is used. For the dithered
scheme described above, either backside or frontside viewing technique
could be used. The reader should be capable of using software algorithms
to frame grab the RGB component images and combine them to display the
color image.

[0158]In the current patent application we have disclosed the technique
where we expose the photoresist using laser lithography process, develop
the photoresist, and than etch into the chrome layer (hard mask). The
reason we are following this technique rather than directly milling using
laser ablation technique into chrome is mainly because the time it takes
to mill directly into metal is much higher than the lithography technique
(at least by a factor of 40) and this will drastically slow down the
whole archival process.

[0159]But as high frequency lasers start coming into the market in the
future it is possible we can directly either write into hard mask or even
in Archival material in a one-step process. However, such techniques
suffer from the following drawbacks:

[0160]1. Low throughput

[0161]2. Quality of the features is not as good as lithography technique.

[0162]3. Involves high energy lasers

[0163]4. Lacks control on precise depth of milling

[0164]Another anticipated technique used for analog data storage is
electron beam lithography. We have explored this area too and it is an
ideal technique for ultra high-density analog store technique. The
process steps are exactly the same as the laser lithography process step.
Due to small spot size, much finer features could be exposed. As far as
comparing electron beam to ion beam, the electron beams are much faster
and superior to ion beam, but for the feature sizes that we are talking
about it is much more economical to use laser lithography techniques.

[0165]Our substrates to start with are metal or metal oxide thin film
coating on glass or quartz or sapphire substrate. One can use focused ion
beams to directly etch the data into the thin metal layer but due to
Gallium ion implantation the glass/quartz/sapphire substrate properties
are changed and it affects the duplication process where UV light is
transmitted through the copy master to generate more copies. Electron
beam assisted etching of metals or metal oxides thin films using gas
chemistry overcomes this problem. Since electron beam doesn't have the
mass to sputter the substrate material we are using corrosive gases and
the etching process is very localized and only happens where the electron
beam impacts the substrate where corrosive gas has been adsorbed.

[0166]Step 1: Inject corrosive gas at a localized spot using a gas
injection needle. For example: halogen gas for metals and xenon
difluoride or metal carbonyls for metal oxide

[0167]Step 2: Let the gas be adsorbed into the substrate, this process
takes few microseconds

[0168]Step 3: Bombard the localized spot with high-energy electron beam.
It initiates the to formation of volatile compounds that are sputtered
away from the substrate leaving the etched pattern we desire.

[0169]Step 4: The corrosive gas adsorption and electron assisted etching
continue as long as the metal or the metal oxide layer is not completely
depleted from the localize spot where the etching is being conducted.

[0170]Another method for ultra-high density analog data storage is the use
of lithography techniques such as SCALPEL, which allow further reduction
in feature sizes on the patterned substrate while still using a mask with
larger patterned features. This technique has been introduced in
semiconductor processing to allow patterning of very small features. A
brief description of the basics is provided here:

[0171]SCALPEL is the acronym for scattering with angular limitation
projection electron-beam lithography. This technology is being touted by
many to be the next generation lithography that will replace optical
techniques from 130 nm down to the 35 nm level. The SCALPEL system
utilizes a beam of electrons as the exposure medium rather than the more
conventional light sources and relies on the contrast caused when they
are scattered at different angles. The mask consists of a membrane of low
atomic number material covered by a patterned layer of high atomic number
material. Although the mask is essentially transparent to electrons at
100 Kev, a high contrast aerial image is generated at the wafer plane as
shown in the accompanying figure. This occurs because very little of the
e-beam energy from the highly scattered patterned portion of the mask is
allowed through the aperture while most of the weakly scattered electrons
from the non-patterned portion of the mask make it through. The magnetic
lens system provides a 4:1 demagnified image at the wafer plane. The fact
that the mask is transparent and that most of the absorption of energy is
at the aperture allows the mask to be relatively immune to thermal
instability problems. The extremely thin film required for the masking
material means that the mask use a grill of struts to minimize the
strain. The images formed by the material between the struts must then be
stitched together at the wafer to complete the imaging process. This is a
significant challenge for the technology. As electron current is
increased, interaction of the scattered electrons can cause blurring of
the image, hence limiting the maximum current and throughput. Significant
wafer heating could also result which would cause expansion and
distortion of the pattern. SCALPEL uses the same type of single-layer
chemically amplified resists currently in use for DUV lithography.

[0172]Even with the potential difficulties to be overcome with this
technology, the market entry costs for the technology could turn out to
be reasonable since the mask, resist and processing technologies are
similar to those used in optical lithography. SCALPEL is compatible with
step-and-scan techniques and therefore holds the promise of good
throughput. In addition, excellent overlay accuracy is achievable
allowing mix-and-match strategies to be employed. Extensive support from
inside the industry makes SCALPEL a strong contender for the 100 nm era
and beyond.

[0173]Another method for forming gray scale images by pit depth variation
in shown in FIG. 16. A narrowly defined range of Zinc Silicate glass
compositions are found to produce High Energy Beam sensitive glass (SG)
that possess the essential properties of a true gray level mask, which is
necessary for the fabrication of three-dimensional microstructures with
one optical exposure in a conventional photolithography process. The
essential properties are: [0174]1. A mask pattern or image does not
possess grainy-ness even when observed under optical microscope at
1000× or at a higher magnification [0175]2. The sensitive glass
(SG) substrate is insensitive and/or inert to photons in the spectral
ranges employed in photolithographic processes, and is also insensitive
and/or inert to visible spectral range of light so that a SG mask blank
and a SG mask are permanently stable under room lighting conditions.
[0176]3. The SG is sufficiently sensitive to ion, electron and laser
(directed Energy beam) beam exposure. [0177]4. The directed energy beam
(either ion, electron or laser) induced optical density is a unique
reproducible function of the energy dosages for one or more combinations
of the parameters of a writer (ion, electron or laser). The parameters of
the energy beam writers include beam acceleration voltage, beam current,
beam spot size, addressing grid size and number of retraces.

[0178]FIG. 16 illustrates a method of fabricating variable pit depth for
analog archival of ultra high resolution gray or color images using SG
substrates for 3D profiling of photoresist and reproducing the
photoresist replica in the substrate with the existing micro fabrication
methods normally used for the production of microelectronics is described
below.

[0179]Since there is no grainy-ness, SG is capable of resolution to
molecular dimensions. SG turns dark instantaneously upon exposure to an
energy beam, the more energy dosage the more it darkens. Therefore SG
glass is ideal for fabricating gray level masks. SG gray level masks can
be written with an ion, electron or laser writer using a 0.1-5 microns
addressing grid size, which is based on the spot size of the energy beam
used. Every spot in the energy sensitive glass substrate 150 acquires a
predetermined transmittance value ranging from 100 percent down to less
than 0.1 percent upon energy beam patterning with a predetermined dosage
for each address. A gray level mask made of SG does not relay on a
halftone method. Therefore, it is a true gray level mask.

[0180]In FIG. 16 in step A, an energy beam is directed to portions 152,
154 and 156 of SG substrate 150 with the amount of energy directed at
each microscopic location within the portions denoted by the arrows 158
with thicker arrows denoting a dose of more energy. The resulting gray
scale photomask created in step B by the energy beam exposure in step A
carries patterns, such as at exposed portions 152, 154, and 156, with
areas of different transmittance. More directed energy causes a reduced
transmittance, as in the far right locations of portions 152, 154, and
156. When the pattern is printed on photoresist 158 in steps C and D,
areas of different transmittance in the gray scale mask create areas of
different thickness in photoresist 158 after development, for example
three-dimensional features 162, 164, and 166 formed on substrate 160.
Therefore, a gray scale pattern in a gray scale photomask can be used to
create predetermined ultra high resolution gray scale image in
photoresist film, which are then transferred into archival quality metal
disk using electroplating technique as by first depositing in step E a
thin conductive layer 168 over the patterned photoresist,
electrodepositing in step F an archival quality metal 170 onto the
conductive layer 168, and separating in step G the archival metal 170
from the patterned photoresist and substrate.

[0181]The energy beam darkening mechanism of SG includes an intermittence
effect in addition to the heat effect. The energy beam darkening
mechanism is not known with certainty and is postulated as follows. In
the presence of a high energy beam, some of the Cl.sup.- ions and
Ag.sup.+ ions in the silver halide complex crystal or complex microphases
in the integral ion exchanged surface glass layer of a SG plate, react
with the energy beam to produce Cl atoms and Ag atoms. Cl atoms and Ag
atoms are not stable species and a reverse reaction takes place
simultaneously. A third reaction process also occurs simultaneously
wherein portions of Cl atoms and Ag atoms become stable species of
Cl2 and Silver specks Agn (n is an integer) with the help of
lattice vibrations. The formation of a silver speck consisting of 2, 3 or
more atoms requires the deformation of silver halide lattice to silver
lattice. Cycles of lattice vibration of sufficient amplitudes are
necessary to cause the formation of the silver specks. Since large
amplitudes of lattice vibrational modes exist at higher temperatures,
silver specks are formed more quickly at a higher temperature. The
variation in the silver speck formation when exposed to high-energy beam
leads to darkness variation in the SG substrate.

[0200]Microfilm/microfiche requires special environmental conditions for
storage of the film. Under strict environmental conditions archival
quality microfilms can have a life expectancy of ˜100 years.
Generally microfilm/microfiche may require re-copying into new films in
as short a span as 30-50 years based on the storage conditions and
handling. Re-copying leads to loss of data due to degradation of
resolution.

[0201]The current microfilm/microfiche technology available for analog
storage for archival purposes does not provide "long-term" (1000+ years)
storage of data on durable media capable of withstanding all forms of
corrosive environment. Additionally, storage of data on microfilms
becomes expensive in the long run due to the requirement for
environmental controls and re-copying at the end of life.

[0202]As previously stated the present PermaFiche process technology can
extend the life of existing Microfilm and Microfiche to 1000+ years. The
process steps for implementing the data transfer from microfilm to a
permafiche substrate are shown in FIG. 17. Here we do not need a focused
energy beam to direct write the data into the photoresist, instead we use
the existing microfilm or microfiche 172, with black regions 174, 176 and
gray-scale region 178 as our mask and shine a UV light 180 for a
predetermined amount of time to expose the photoresist 182 below the
microfilm. Based on the type of photoresist used (positive or negative)
the exposed regions will either get washed off or remain during the
developing process (FIG. 17 assumes a positive photoresist), thereby
forming step features 184, 186 and ramp features 188. The standard
semiconductor lithography exposure setup and technique is used. One also
has the additional advantage of reducing the page dimension in the
original microfilm by a factor of 5, 10 or higher to obtain higher
storage density as by using a reduction lens 190. The process steps are
as follows:

[0203]Step 1 (B): Expose photoresist 182 in an optical lithography tool
using the Microfilm or Microfiche 172 as the mask. Preferred UV exposure
parameters include using light have a peak spectral emission at 200-500
nm, exposing the photoresist 182 for between about 100 and 500 watts with
200 watts most preferred, for an exposure time of 20 seconds to 2 minutes
with 30 seconds most preferred. Exposure can be 1:1 or reduction lens 190
could be used to reduce the physical size of the analog data.

[0204]Step 2 (C): In FIG. 17 positive photoresist is used as an example.
The exposed regions are removed when developed leaving features 184, 186,
188 on substrate 200. Also if the microfilm or microfiche 172 had true
gray scale image 178 we will obtain a etch gradient during the developing
process as shown by ramp feature 188. This helps preserve the original
data quality in the microfilm or microfiche and stores data in true gray
scale.

[0205]Step 3 (D): Prior to the electro-deposition process a thin
conductive metal layer 192 is deposited on the top surface either using
sputtering or electroless technique.

[0206]Step 4 (E): Archival quality metal 194 is electrodeposited.

[0207]Step 5 (F): The electrodeposited metal layer 194 is then separated
out from the substrate 200 to reveal negative features 196, 197, 198
corresponding to positive features 184, 186, 188. The resulting archival
metal disk 194 is cleaned in acetone, followed by a thorough rinse in
isopropanol and blow dried with nitrogen gas.

[0208]Having described and illustrated the principles of the invention in
a preferred embodiment thereof, it should be apparent that the invention
can be modified in arrangement and detail without departing from such
principles. While the above description describes processes for analog
data storage for archival purposes, the same process methods can be used
for inscribing analog data into any substrate, archival or other, such as
jewelry products--diamonds and gemstones, gold pendants, watches, rings,
bangles, bracelets--and so forth for the purpose of marking a brand or
including other identifying information. An example would be that the
processes described herein are used to create jewelry products having
inscribed religious texts thereon, such as the bible inscribed on a cross
hung as a pendant. I thus claim all modifications and variation coming
within the spirit and scope of the following claims.